Spatial variability of summertime tropospheric ozone over the continental United States: Implications of an evaluation of the CMAQ model

Abstract

This study evaluates the ability of the Community Multiscale Air Quality (CMAQ) model to simulate the spatial variability of summertime ozone (O 3) at the surface and in the free troposphere over the continental United States. Simulated surface O 3 concentrations are compared with 987 Air Quality System (AQS) sites and 123 Clean Air Status and Trends Network (CASTNet) sites. CMAQ's ability to reproduce surface observations varies with O 3 concentration. The model best simulates observed O 3 for intermediate concentrations (40–60 ppbv), while over-(under-) predicting at lower (higher) levels. CMAQ reproduces surface O 3 for a wide range of conditions (30–80 ppbv) with a normalized mean error (NME) less than 35% and normalized mean bias (NMB) lying between ±15% for the whole domain. Although systematically over-predicting O 3 in the east and under-predicting it in the western United States, CMAQ is able to reproduce 1- and 8-h daily maxima with a cross-domain mean bias (MB) of 1 and 8 ppbv, or NMB of 8% and 25%, respectively. The model underestimates observed O 3 at rural sites (MB=−5 ppbv, NMB=−5% and NME=23% with a 40 ppbv cut-off value) and over-predicts it at urban and suburban sites by a similar magnitude (MB=6 ppbv, NMB=7% and NME=25%). Apparent errors and biases decrease when data is averaged over longer periods, suggesting that most evaluation statistics are dependent on the time scale of data aggregation. Therefore, performance criteria should specify an averaging period (e.g., 1- or 8- h) and not be independent of averaging period as some current model evaluation studies imply. Comparisons of vertical profiles of simulated O 3 with ozonesonde data show both overestimation and underestimation by 10–20 ppbv in the lower troposphere and a consistent under-prediction in the upper troposphere. Vertical O 3 distributions are better simulated when lateral boundary conditions obtained from the global Model of Ozone and Related Tracers version 2 (MOZART-2) are used, but under-prediction remains. The assumption of zero-flux at the top boundary and the resulting exclusion of the contribution of stratosphere–troposphere exchange to tropospheric O 3 concentrations limit the ability of CMAQ to reproduce O 3 concentrations in the upper troposphere.